Two-step excitation photocatalytic systems called Z-scheme systems are attractive to achieve overall water splitting under visible light irradiation. The low activity of SrTiO3:Rh as a H2-evolving photocatalyst suppresses the overall efficiency of the Z-scheme system composed of SrTiO3:Rh modified with a Pt or Ru cocatalyst, BiVO4, and an Fe3+/Fe2+ electron mediator. In this study, improvement of efficiency for water splitting over the Z-scheme system has been achieved through the preparation of highly efficient SrTiO3:Rh photocatalysts by hydrothermal and polymerizable complex methods. It has also been revealed that excess amounts of Sr are necessary to obtain highly active SrTiO3:Rh photocatalysts. Apparent quantum yields of the Z-scheme systems were improved from 0.4% to 3.9-4.2% at 420 nm when SrTiO3:Rh prepared by HT and PC methods was used as a H2-evolving photocatalyst instead of that prepared by a solid state reaction. The efficiency of the improved Z-scheme system for solar energy conversion was determined to be 0.1%.